Nitrocellulose propellants containing a dinitrile plasticizer



Sept 17, 1963 F. M. ERNSBERGER ETAL 3,104,190

NITRocELLULosE PROPELLANTS CONTAINING A DENITRILE PLASTICIZER Filed oct. 24, 1957 FRED M. ERNSBERGER MARTIN H. KAUFMAN ATTORNEYS United States i Patent O 3,ltl4,1tl NTRGCELLULOSE PRGPELLANTS CGNTAlNlNG A DENITRELE PLASTECHZER Fred M. Ernsberger, Pittsburgh, Pa., and Martin H. Kaufman, China Lake, Calif., assigner-s to the United States of America as represented by the Secretary of the Navy p Filed Oct. 24, 1957, Ser. No. 692,239

7 Claims. (Cl. 149-98) Y (Granted under Title 35, US. Code (1952), sec. 265) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to new propellent formulations,`V

more particularly, it relates to propellent formulations having cyanogen compounds incorporated therein as plasticizers.

Carboxylic esters have been used in the past almost exclusively as the non-explosive plasticizer for nitrocellulosebase propellants. The most common plasticizing agent of this type is diethyl phthalate. These and other esters which have been used, such as, certain linear esters of which diethyl phthalate is representative, contribute nothing energy-wise to the propellant. Since the plasticizing agent often contributes up to 15% of the total weight of the propellant it would be highly desirable to have available a plasticizing agent which contributes energy to the propellant in addition to a plasticizing elfect.

It is therefore an object of this invention to provide propellent formulations incorporating non-explosive plasticizing agents which provide energy to the propellant.

It is another object of this invention to provide nonexplosive plasticizing agents for nitrocellulose-base propellants which are compatible with the propellants, provide the required plasticizing action and do not adversely affect the ballistic properties of the propellant.

It .has been found that the above and other objects are accomplished by incorporating into the nitrocellulose-base propellant up to about 15% of a dinitrile containing up to twelve carbon atoms.

The invention is best understood -by reference to the Vfollowing description and the accompanying drawing hereby made a part of this specication and which is a logarithmic graph of the pressure burning rate relationship of a number of` propellent formulations made by incorporating typical dinitriles into N- propellant as inert plasticizers. The graphs were made by plotting burning rate against pressure.

The dinitriles possess a number of properties which make them peculiarly suitable as plasticizers for nitrocellulose-base propellants. Based on equal oxygen consumption, the nitrile group is more energetic than the ester group by about 12 kilocalories per gram-molecular weight.

f The nitrile function (-CN) has a small cross-section and an ester plasticizer.

has a positive heat of formation of approximately 30 kcaL/mole. As a consequence, on an equimolar basis a dinitrile will lower the heat of explosion less than Other advantages accrue from the low volatility of dinitriles, for example, pimelonitrile containing seven carbon atomsis less volatile than dimethyl azelate which has eleven carbon atoms. Also, tests have indicated that the linear dinitriles are slightly superior to the `linear diesters in low-temperature performance. It is also true that the dinitriles as a class present a favorable picture as respects chemical stability and toxicity. In general, the dinitriles have a high solvating action for nitrocellulose, indicating the possibility that greater tensile strength may be achieved, with nitrocellulose propellants, by using dinitriles as the plasticizer for such systems.

3,l0-4,l9 Patented Sept. 17, 1963 ICC Tests were run on a number of representative dinitriles to determine such properties as viscosity, vapor pressure and solubilizing eect on nitrocellulose, these properties being important in a plasticizer for nitrocellulose-base propellants. v

The following table presents comparative viscosity data for typical dinitriles and two conventional nitrocellulose plasticizers, diethyl phthalate and acetone.

1 Intrinsic viscosity number. 2 Slope o the line obtained by plotting concentration against the quotient of specific viscosity divided by concentration.

The above results indicate that the dinitriles as a class are good solvents for nitrocellulose. They show that adiponitrile, for example, is a better solvent for nitrocellulose than even acetone. ln the solvent type plasticizers such as the dinitriles this is important.

Vapor pressures of representative dinitriles and diethyl phthalate are shown in the following table, in microns at the temperatures indicated. Vapor pressures were determined by the method of Ernsberger and Pitman as disclosed in the article, New Absolute Manometer for Vapor Pressures in the Micron Range, Review of Scientific Instruments, volume 26 (1955), pages 5 88-5 89.

The above results indicate that the vapor pressures of the dinitriles compare favorably With those of conventional plasticizers.

Nitrocellulose-base propellent formulations with which the dinitriles have been found to be eifective as plasticizers are represented generally by a propellant known as N-S having the following composition given in percentage ranges of the components.

T able 3 Wt. percent Component: (range) Nitrocellulose 44-60 Nitroglycerin 28-47 Non-explosive plasticizer 3-15 Stabilizer 1-5 Ballistic modifier f 2 5 The non-explosive plasticizer may be diethyl phthalate, dibutyl phthalate, dimethyl sebacate or the dinitrile plasticizers of this invention. Stabilizers may be symmetrical diethyl diphenyl urea or 2-nitrodiphenylamine. The ballistic modifier may be lead salicylate, lead acetylsalicylate aloe-,iso

or lead 2,4-dihydroxybenzoate. This propellant is being used in large quantities by the Armed Forces in the 2.75" rocket known as the Mighty Mouse and disclosed in US. Patent Number 2,801,587. Formulations of the above propellant were made by incorporating into them dinitrile plasticizers instead of the usual non-explosive plasticizers, and the formulations subjected to tensile strength, brittle point, second order transition, burning rate and stability tests. In the majority of the tests reported the formulations are referred to as examples as shown below.

Table 4 Nitro- Plasti- Calcu- Calcuglyeerin eizcr lated lated Example Plastleizer (Wt. (Wt. Hillier H.E.ifor

perperplastieizer, Compoeent) cent) cal./gm. sition.

caL/gm.

I (N-) Diethylphthalate. 34.8 10.6 -1,760 855 Il Suecinonitrile. 34.0 10.6 1.271 -878 III.- Adiponltrie 85.4 10.0 1,820 -867 IV do 30.4 10.0 4,820 825 V. Pimelonitrlle 35.4 10.0 -1,905 -850 VI Sebacenllrile 30.1 0.3 2,350 842 1 Heat of explosion.

With the exception of Example IV all of the compositions contained 50% `nitrocellulose (12.6% N), 1.2% lead salicylate, 1.2% lead Z-ethylhexoate, 2.0% 2-nitrodyphenylam-ine, and 0.2% candelilla wax (added). Example i1V contains 55% nitrocellulose.

The propellent formulations were made by standard processes as illustrated by the follow-ing. They were mixed 'in a Baker-Perkins water-jacketed Sigma-blade rrrixer in forty gram lots. The nitrocellulose was wet with ethanol (21.9% on a weight basis); the nitroglycerin was desensitized with acetone (20-30%). The nitrocellulose, nitroglycerin, stabilizer, plastioizer, and additives were blended in the mixer with enough additional acetone to bring the solvent-to-dry-nitrocellulose ratio to 0.8: 1. The acetone and alcohol were in a 65-35 weight ratio. These formulations were mixed for a minimum ofjfive lhours, partially dried and extruded into strands %2 inch in diameter, using a remote-controlled, `hydraulically-driven, ram press. The extruded strands were then put through a four-day drying cycle to give an essentially solvent-free, well-colloided propellant. The strands used for burning rate studies were inhibited by the process described in NAVORD OD 9376, May 29, 1953, Standard Methods and Procedures for Strand Burning Rate Evaluation of Rocket Propellant Powder, Bureau of Ordnance.

Tensile strength to the break point was measured at 70 F. for a number of eight inch sections of the dried, cured, extruded strands using ya Dillon tensile testing mach-ine at a loading rate of two inches per minute. The percent elongation at the break point was noted. N-5 propellent strands made by either a standard solvent process or by the standard slurry technique had tensile strengths ranging from about 500 Ito 700 p.s.i. with elongation of about 2%. N-5 formulations modified by the substitution of adiponitrile for diethyl phthalate as the inert plastieiner (Example IV) had tensile strengths ranging from 3000 to 4000 psi. and elongations between -20%. Strands made from the formulation of Example yIilI had tensile strengths of about 1600 p.s.i. and gave elongations of 15-20%.

The advantage of dinitriles over conventional non-explosive plasticizers for nitrocellulose-base propellants is clearly indicated by the tensile strength data above, where it is shown that a 6- to 8-fold increase in tensile strength was eil'ected by the use of dinitrile plasticizers.

Tensile strength tests were made using films containing essentially only nitrocellulose and either the dinitrile plasticizers or diethyl phthalate. The films were made as follows: Fve grams of dried nitrocellulose (12.2% nitrogen) and plasticizer (enough to give either 0.0044

Table 5 Moles, 5 Weight, Volume Tensile Plasticizer g. of percent Plasti- Strength,

NC 1 Plastieiser p.s.i.

eizer (average) Adiponitrile 0. 0044 8. 7 14. 2 10, 603 Do 0 0088 10.0 24. 9 0,188 Sebacontrtle.. 0 0044 12. 6 20. -t 10, 727 Do 0 0088 22.4 33.8 5, 900 Dlethyl Phthalate 0. 0044 15.9 20. 8 8, 850

1 Nitreeellulose.

Second-order transition temperatures were measured For a numb-er of formulations to arrive at comparative data on the brittle point temperature of standard N-5 propellant and that of formulations made by substituting representative dinitrfiles for the standard inert plasticizer in N-S compositions. As the temperature is lowered, double-base propellants gradu-ally harden and become brittle. Below a certain temperature they tend to shatter upon rough handling or from the etiect of igniterblast shock, This temperature is known as the brittle point and is an important factor in determining the lower temperature limit of serviceability for the propellant.

The second-order transition temperature of a solid amorphous high polymer is closely related to the brittle point and can be located by nondestructive measurements. At the second-order transition temperature, the therma -expansion coeicient, the temperature coefficient of refractive index, and the specific heat all change discontinuously, and the dielectric loss factor exhibits a maximum. The second-order transition temperatures observed in the thermal-expansion coeliicient, the temperature coeti'icient of refractive index, and the speciiic heat all depend on small, very slow deformations that are interpreted by Boyer and Spencer in the case of thermal expansion as due to a viscous-flow mechanism. (Thermal Expansion and Second-Order Eeots in High Polymers; Part Il; The/Ory, Journal of Applied Physics, volume 16, page 594, 1945.) The brittle-point tests involve relatively large and rapid deformations as cornpared with the thermal-expansion tests, and the results are interpreted as a lhigl'i-elasticity (chain uncoiling) mechanism, which may be further complicated by superposed viscous flow. The diielectric-loss-factor test is interpreted as dependent on rapid rotation of dipoles and involves no gross displacements of the sample. The relationship between these tests has been pointed out by Boyer and Spencer, who say that each of these mechanisms requires rotation of chain segments about carboncarbon bonds. This segmental rotation permits viscous flow, elastic deformation, or dipole rotation, depending on how much and how frequently the segments rotate. From the data summarized by the above writers it seems to be generally 4true that the thermal-expansion transition is the lowest in temperature. The lbrittle point is higher, but its value approaches that of the thermal-expansion transition temperature if the method of measurement involves small, slow deformations. The dielectric-loss-factor temperature is also higher than the thermal-expansion transition temperature, with the actual value dependent upon the frequency used. Accordingly, the secondorder transition is a manifestation of the same physical changes occurring in the high polymer which causes brittleness, and factors which change the brittle temperature will also change the transition temperature.

` In arriving at lthe second-order transition temperatures adaptations of ASTM stand-ard methods of measuring dielectric constants and power factors were used to measure dielectric'loss factors. These methods are fully set forth in the article, Tentative Methods of Test for Power Fac-tor and Dielectric Constant of Electrical Insulating Materials in ASTM Standards, Part III-B, Nonrnetallic Materials; ASTM Designation DISOL46T; Philadelphia,

Pennsylvania; American Society for Testing lvlaterials,`

1946, pages 647-681. Films for testing were cast from acetone solutions. These films were dried for at least forty-eight hours before the second-order transition measurements were made. The following second-order transition temperatures were obtained from tests made on the compositions shown. Compositions are given in weight percents. Temperatures are given in degrees centigrade.

The results show that the dinitriles are about as good as the better diester plasticizers in imparting low-temperature exibility to nitrocellulose compositions.

Second-order transition measurements were made on lms prepared from the compositions of Examples I, Il, IV, V and Vl set forth in Table 4. These lms were made as follows: Broken pieces of strands were piled together and pressed between chromeplated steel sheets using steel spaces in a vCarver press at a temperature of 90-95 C. and a pressure of 3000 lbs/in.2 for 4 minutes. The -sandwich was then removed and cooled quickly by pressing on a cold metal surface. The stripped film was then folded into a cubical shape and the pressing process repeated once. These films were approximately 20 mils thick, tough, flexible, and transparent or translucent. After being out to constant diameter with a cork borer they were ready for brittle-point measurement in the dielectric apparatus. The measurements obtained are set forth in the following table in degrees centigradc.

Table 7 Example Plasticizer Temperature Dlethyl Ihthalate -27 .5 Succinonitrile -28 Adipnnitrile -26 Pimelonitrile. -28 Sehaconitrlle -31 The results show that the dinitrile additives do not adversely affect the stability of nitrocellulose-base propellauts.

A required characteristic of any additive to a propellent formulation is that lit must not prohibitively alfect the ballistic properties of the propellant, and particularly, the pressure-burning rate relationship of the propellant. This relationship is dened by the slope n, or pressure exponent, of the curve produced by a logarithmic graph of the burning rate of the propellant plotted against pressure. The relationship is particularly impontantin the zone of useful rocket pressures, roughly, 7604000 p.s.i. The relationship between the pressure at which a propellant burns and its burning rate is mathematically expressed as r=cpn or as log r=n log p-i-log c, Ewhere r is the burning rate, p is the pressure at which the burning rate is measured, and c and n are constants characteristic of a given propellant. A plot of log r against log p for conventional propellants, for example, produces a straight line of slope n, that is, there is a progressive increase in burning rate for each increase in pressure.

N-S is la lmesa-type propellant, that is, it is characterized by the fact that the slope of the curve representing its pressure-burning rate relationship becomes Zero at some point and then reaches a negative value thereafter in fthe region of useful rocket pressures so that the burning rate in this negative slope region actually decreases with increase in pressure. This phenomenon is due to the presence of certain lead compound bdlistic modifiers in critical amounts. The phenomenon results in a number of advantages. For example, the negative pressure exponent serves as a safety valve in case of sudden large changes in burning surface during the operation of a rocket, such as that caused by cracking of the grain. With other type propellants such a failure of the grain would ordinarily result in destruction of the rocket motor, but in the case of the mesa-type propellant only a small pressure increase results. Funther, in the mesattype propellant, there is an inherent tendency for overlapping of rate-pressure relationships at various temperatures as illustrated by logarithmic graphs of the relationships, that is, in certain regions of pressure the burning rate of a propellant for iirings at low temperature may actually be higher than the burning rate for rings at high temperature. Further, the variation in performance with change in temperature for mesa-(type propellants is negligible and in some cases there is none at all. This advantage is particularly useful as respects fire control considerations in the design of aircraft rockets to be fired at moving targets. Further, the burning ratte and energy content of the mesa-type propellants are higher than those of other type propellants and can be controlled at will over rather wide limits. The mesa characteristic is highly sensitive fro the addition of even small amounts of additives to the propellent formulation so that it is a required property of the additive, whether it be a stabilizer, plasticizer, etc., that it not vitiate the mesa characteristic of the propellant. This is equally true as respects corresponding ballistic properties of other propellants to which Ithe ingredient may be added.

Burning rate .tests were made on N-S formulations (Examples H, III, V and VI) in which the standard ballistic modifier, diethyl phthalate, had been replaced by a corresponding amount of a dinitrile. The 3/32 inch strands described above were used. The method used was the described in the anticle, Direct Determination of Burning Rate of Propellant Powders, B. L. Crawford, 5r., and others, Analytical Chemistry, volume 19 (1947) pp. 630-633. A standard pressure-burning rate curve as described above was plotted for each example, and is presented in the accompanying drawing. The corresponding curve (average a ea) for N-S propellant is represented in the drawing by the shaded area. it is to be noted .that although small aberrations from the N-S curve result from the addition of the dinitriie plasticizers the mesa characteristic is substantially unaffected so that the compounds are satisfactory in this respect.

While the invention has been illustrated by its application -to N-5 propellant it is not limited thereto as the dinitrile plasticizers are equally effective with all nitrocellulose-base propellants. They are effective for plasticizing nitrocellulose alone. They may be used in nitrocellulose-base propellants in a range from 3 to 15 weight percent of the propellant. ln using them they are thoroughly incorporated into the propellent mix by conventional mixing techniques as described above. The prcpellants themselves are used in rockets of the type described in U.S. Paten-t Number 2,801,587. The dinitriles which are operative are not limited to those disclosed as these are merely representative of this class of compounds. Other related compounds which have been found effective are certain -eyanoethyl esters of aliphatic dicarboxylic acids, derivatives of w-cyanopelargonic acid and ot-w dinitriles with miscellaneous hetero atoms or groups in the chain.

It is seen from the test results presented above that the dinitriles are effective plasticizers for nitrocellulosebase propellants. Their properties as respects viscosity and vapor pressure are satisfactory. Nitrocellulose-base formulations incorporating the dinitrile plasticizers exhibit satisfactory properties as respects brittle point, stability, and superior tensile strength properties.

cp u

They do not undesirably affect the ballistic properties of nitrocellulose-base propellants.

Gbviousiy, many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

l. Propellent formulations, consisting essentially of about forty to sixty-five percent of nitrocellulose, from about twenty-five to about iity percent of nitroglycerin and from about five percent -by weight to about fifteen percent by weight of a dinitrile having up to twelve carbon atoms in the molecule.

2. A nitrocellulose formulation for use in propellants consisting essentially of about five percent to about 40 percent by weight of a dinitrile having up to 12 car-bon atoms in the chain and the remainder nitrocellulose.

3. The formulation of claim 2 in which the dinitrile is adiponitrile.

4. The formulation of claim 2 in which the dinitrile is suberonitrile.

5. The formulation of claim 2 in which the dinitrile is azelonitrile.

6. The formulation of claim 2 in which the dinitrile is sebaconitrile.

7. The formulation of claim 2 in which the dinitrile is pimelonitrile.

References Cited in the file of this patent UNITED STATES PATENTS 2,417,090 Silk et al. a Mar. 1l, 1947 OTHER REFERENCES Billmeyer, Textbook of Polymer Chemistry, Interscience Publishers, Inc., New York, pages 285-6. Received in Scientific Library May 6, 1957. 

1. PROPELLENT FORMULATIONS, CONSISTING ESSENTIALLY OF ABOUT FORTY TO SIXTY-FIVE PERCENT OF NITROCELLULOSE, FROM ABOUT TWENTY-FIVE TO ABOUT FIFTY PERCENT OF NITROGLYCERIN AND FROM ABOUT FIVE PERCENT BY WEIGHT TO ABOUT FIFTEEN PERCENT BY WEIGHT OF A DINITRILE HAVING UP TO TWELVE CARBON ATOMS IN THE MOLECULE. 